Linear organosiloxane polymers as typified by silicone oils are generally organosilicon compounds possessing a straight or branched chain structure of siloxane bonds and having organic groups attached to silicon atoms, typically alkyl and aryl groups. They have been widely utilized in the industry because their viscosity can be adjusted over a wide range and because they largely differ in heat and freeze resistance, viscosity change with temperature, and living body activity from ordinary organic compounds such as mineral oils, animal oils and vegetable oils.
These linear organosiloxane polymers are generally prepared by either of two known processes, equilibration and polycondensation processes. The equilibration process, which is most commonly employed in preparing linear organosiloxane polymers, uses acidic or basic materials as catalysts to yield an equilibrated polymer product of linear polymers and cyclic oligomers. Since the polymer obtained by this process contains about 15% of cyclic oligomers, linear polymers must be isolated as by stripping, giving rise to undesired problems such as an extended process time and an increased cost.
On the other hand, the polycondensation process is to produce linear organosiloxane polymers by starting with siloxane monomers or oligomers and promoting condensation of end groups in the presence of suitable condensation catalysts. A variety of condensation catalysts are known in the art, for example, phosphorus nitride chlorides as described in U.S. Pat. Nos. 5,380,902 and 6,136,996 and relatively weakly acidic or basic catalysts. Phosphorus nitride chlorides (PNC) are allegedly preferred. The condensation process is advantageous over the equilibration process in that no cyclic oligomers are produced. Products of high purity impose only a little stripping burden, and some products of fully high purity are ready for use without a need for stripping. However, the condensation process is applicable to only the preparation of hydroxy-terminated oils and not applicable to the preparation of other linear organosiloxane polymers, that is, linear organosiloxane polymers terminated with saturated or unsaturated hydrocarbon groups or aryl groups.
Under the above-described circumstances, there have been a need to have a process capable of linear organosiloxane polymers without limits on terminal substituent groups and with no or little formation of cyclic oligomers. A number of research works have been made therefor.
For example, U.S. Pat. No. 5,420,221 discloses a process for producing a linear organosiloxane polymer by disproportionating or condensing and disproportionating a mixture of two organosiloxanes having different molecular weights, using a mixture of a phosphorus-nitrogen homopolymeric or oligomeric compound, sulfuric acid, fluorinated alkane sulfonic acid, and kaolin clay, without increasing the cyclics content. This process is successful in producing the desired linear organosiloxane polymer without limits on the type thereof and without forming substantial quantities of cyclics.
However, phosphorus nitride chlorides used as the catalyst in the process are less stable to water, for example, readily hydrolyzable with air-borne moisture or water formed during polycondensation, substantially losing catalysis. Therefore, the phosphorus nitride chlorides are hardly regarded as easy to handle from the industrial standpoint, and in particular, difficult to use in reactions using silanol-terminated organosiloxanes which are of interest as starting reactants. Also, fluorinated alkane sulfonic acids as typified by trifluoromethanesulfonic acid used in the process are difficult to handle because of their strong acidity and high toxicity. An additional problem arises from kaolin clay which is difficult to purify, with the increased risk of impurities being leached out to contaminate the product, requiring a separate impurity removal step.
JP-A 10-511996 discloses a process for producing a polyorganosiloxane free of cyclics, using phosphorus nitrile chloride and perfluoroalkylsulfonic acid or using the reaction product of phosphorus nitrile chloride with silyl phosphate. Like the process described just above, this process can produce a desired linear organosiloxane polymer without forming cyclic oligomers. As used therein, phosphorus nitrile chloride is highly susceptible to hydrolysis, and perfluoroalkylsulfonic acid is strongly acidic and highly toxic, and both are difficult to handle. When the reaction product of phosphorus nitrile chloride with silyl phosphate is used, the catalyst is improved in stability, but takes a long time to prepare it and is expensive. Consequently, the process is uneconomical.